1. Technical Field
Apparatuses consistent with the present invention relate to an automatic transmission for changing the revolutions of an input shaft driven rotationally by a prime mover or the like of an automobile, to a plurality of gears.
2. Description of Related Art
In JP-A-2007-24303 (Paragraphs [0715]-[0718], and FIG. 125 and FIG. 126) (hereinafter “Patent Document 1”), there is disclosed an automatic transmission comprising: an input shaft; a speed-reducing dual planetary gear including a speed-reducing common carrier fixed in a transmission case for bearing a stepped pinion, a first sun gear meshing with the diametrically-larger pinion portion of the stepped pinion and connected directly to the input shaft, a second sun gear meshing with a diametrically-smaller pinion portion, and a speed-reducing common ring gear meshing with the diametrically-smaller pinion portion; a speed-changing dual planetary gear including a third carrier for bearing a pair of pinions meshing with each other, a third sun gear meshing with one pinion of the paired pinions, a third ring gear meshing with the other pinion, a fourth carrier connected directly to the third ring gear for bearing the pinions, a fourth sun gear meshing with the pinions and connected directly to the third carrier, and a fourth ring gear meshing with the pinions and connected directly to an output shaft; an output shaft connected directly to the fourth ring gear; a first clutch for connecting the second sun gear and the third sun gear selectively; a second clutch for connecting the fourth carrier selectively to the input shaft; a third clutch for connecting the second sun gear selectively to the third carrier; a fourth clutch for connecting the fourth sun gear selectively to the input shaft; a fifth clutch for connecting the first ring gear selectively to the third carrier; a first brake for fixing the third carrier selectively; and a second brake for fixing the third ring gear and the fourth carrier connected directly, selectively, whereby the revolutions of the input shaft are changed to forward ten gears and reverse two gears and transmitted to the output shaft.
In JP-A-2007-57084 (Paragraphs [0118]-[0120], and [0149]-[0163], and FIG. 15 and FIG. 18) (hereinafter “Patent Document 2”), on the other hand, there is disclosed an automatic transmission comprising: an input shaft; a speed-reducing dual planetary gear including a first carrier fixed in a transmission case for bearing a first pinion, a first sun gear meshing with the first pinion, a first ring gear meshing with the first pinion, a second carrier bearing second and third pinions meshing with each other and connected directly to the input shaft and the first sun gear, a second sun gear meshing with a second pinion and connected directly to the first carrier, and a second ring gear meshing with the third pinion; a speed-changing dual planetary gear including speed-changing common carrier for bearing a long pinion and a fourth pinion meshing with the long pinion rotatably and for being connected to the input shaft, a third sun gear meshing with the long pinion, a fourth sun gear meshing with the fourth pinion, and a speed-changing common ring gear meshing with the long pinion; an output shaft connected directly to the speed-changing common ring gear; a first clutch for connecting the second ring gear and the fourth sun gear selectively; a second clutch for connecting the input shaft and the speed-changing common carrier selectively; a third clutch for connecting the second ring gear and the third sun gear selectively; a fourth clutch for connecting the second carrier and the third sun gear selectively; a fifth clutch for connecting the first ring gear and the third sun gear selectively; a first brake for fixing the third sun gear selectively; and a second brake for fixing the speed-changing common carrier selectively, whereby the revolutions of the input shaft are changed to forward ten gears and reverse two gears and are transmitted to the output shaft.
In recent years, multiples gears are desired for an automatic transmission to improve the mileage and the power transmission efficiency or to acquire the gear ratios matching the tastes of the driver. These desires for the automatic transmission are addressed by the automatic transmissions disclosed in Patent Documents 1 and 2, which can acquire the gear ratios of forward ten gears and reverse two gears. In the automatic transmission described in Patent Document 1, however, the speed-changing dual planetary gear is constituted to include a third carrier for bearing a pair of pinions meshing with each other, a third sun gear meshing with one pinion of the paired pinions, a third ring gear meshing with the other pinion, a fourth carrier connected directly to the third ring gear for bearing the pinions, a fourth sun gear meshing with the pinions and connected directly to the third carrier, and a fourth ring gear meshing with the pinions and connected directly to an output shaft. As a result, the automatic transmission has its axial length enlarged and its parts number increased to raise the cost. In the automatic transmission disclosed in Patent Document 1, moreover, as apparent from the gear ratios of the individual gears shown in FIG. 126, the differences are large between the individual steps, which are calculated by dividing the gear ratios of n gears by the gear ratios of (n+1) gears. The step between the first gear and the second gear is so high at 1.511 as may cause speed-changing shocks. The step between the ninth gear and the tenth gear is at 1.131 so close to 1 as may cause a poor speed change. Thus, the driver cannot attain a preferable feeling of little dispersion all over the ten gears.
In the automatic transmission described in Patent Document 2, on the other hand, the speed-reducing dual planetary gear is constituted to include a first carrier fixed in a transmission case for bearing a first pinion, a first sun gear meshing with the first pinion, a first ring gear meshing with the first pinion, a second carrier bearing second and third pinions meshing with each other and connected directly to the first sun gear and the input shaft, a second sun gear meshing with a second pinion and connected directly to the first carrier, and a second ring gear meshing with the third pinion. As a result, the automatic transmission has its axial length enlarged and its parts number increased to raise the cost. In the automatic transmission disclosed in Patent Document 2, moreover, as apparent from the gear ratios of the individual gears shown in FIG. 18, the differences are large between the individual steps. The step between the first gear and the second gear is so high at 1.688 as may cause speed-changing shocks. The step between the ninth gear and the tenth gear is at 1.122 so close to 1 as may cause a poor speed change. Thus, the driver cannot attain a preferable feeling of little dispersion, as might otherwise result from the speed change, all over the ten gears.
In order to address demands for reducing the size and weight, for matching the lower cost and for achieving multiple gears while spacing the gear ratios of adjoining gears properly, therefore, the has been a long-felt but unresolved need to develop an automatic transmission which is simple and compact in constitution and which can change the gears with a comfortable feeling.